In the past several decades, silicon-based microelectronics have dominated the electronics industry, and provided the industry nearly constant exponential growth in device capability. However, it is unlikely that silicon-based microelectronic technology can maintain past advancement rates for more than a decade or two longer. Fundamental physical limitations, which prevent current designs from functioning reliably at the nanometer scale will likely be reached, and rapidly rising fabrication costs will make it prohibitive to increase integration levels.
New solutions are being developed to transform electronic devices to ever smaller dimensions, and in particular to circuits constructed using nanometer-scale devices. Molecular electronics can in principle overcome many of the limitations of silicon technology because it is possible to have single-molecule devices that are organized cheaply in parallel by self-assembly. Much of the current development focuses on two key areas of device design: the “wires” or primary conductive or semiconductive paths to be used in the nanometer-scale devices, and the techniques for forming interconnections between such wires.
Two promising nanometer-scale wire technologies are conducting or semiconducting (e.g., silicon) nanowires, and carbon nanotubes. Note that throughout this application, the terms “wire” and “wires” will be used to generally refer to both nanotubes and nanowires.
A number of technologies are being developed to grow nanowire structures, and particularly silicon nanowires which are typically on the order of nanometers in width and can be grown or assembled into sets of long parallel nanowires. The electrical properties of such wires can generally be controlled by selection of the underlying material, and/or by the selection of dopants. Various different nanowire structures and fabrication techniques are well known in the art, and examples can be found in Kueckes et al., U.S. Pat. No. 6,128,214; Morales et al., “A Laser Ablation Method for Synthesis of Crystalline Semiconductor Nanowires,” Science, 279:208–211, 1998; and Collier et al., “Electronically Configurable Molecular-Based Logic Gates,” Science, 285:391–394, 1999; each of which is hereby incorporated herein by reference, in its entirety.
Carbon nanotubes are cylindrical molecules with a diameter of as little as one nanometer and a length typically up to many microns. They comprise carbon atoms and, in the single-walled configuration, can be thought of as a monolayer of graphite wrapped into cylindrical form. Carbon nanotubes can exhibit unique electronic, mechanical, and chemical properties that make them attractive building blocks for molecular electronics. Depending on diameter and helicity, these nanotubes behave as one-dimensional metals or as semiconductors which, by virtue of their great mechanical toughness and chemical inertness, represent good materials for creating reliable, high-density input/output (I/O) wire arrays. Carbon nanotubes have even been used to make specialized field effect transistors. Additionally, the growth and alignment of these nanotubes can be controlled such that they can be assembled into parallel (or roughly parallel) rows of conductors and layered into arrays. Moreover, nanowires can be, in principle, assembled along with nanotubes when their respective properties complement each other. Carbide nanotubes, i.e., nanotubes made from carbon and one or more other elements, doped carbon nanotubes, and nanotubes formed from other materials can also be implemented. Various different nanotube structures and fabrication techniques are well known in the art, and examples can be found in Lieber et al., U.S. Patent Application No. 20030089899; Rueckes et al., “Carbon nanotube-Based Nonvolatile Random Access Memory for Molecular Computing,” Science, 289:94–97, 2000; and Dekker, “Carbon Nanotubes as Molecular Quantum Wires,” Physics Today, 52 no. 5, 22–28, 1999; each of which is hereby incorporated herein by reference, in its entirety.
Many different schemes for constructing nanometer-scale devices using nanotubes and/or nanowires have been disclosed. Most of those techniques use some mechanism to establish an electronic connection or junction between separate wires, typically oriented at some non-zero angle with respect to each other. The most promising mechanisms are repeatable and at least bi-stable, allowing for at least two programmable states, e.g., an ON state and an OFF state. In this manner, programmable nanometer-scale interconnects can be formed and arrays of such interconnects can be used to construct circuit devices.
FIGS. 1A and 1B illustrate prior art examples of such interconnects where the interconnect mechanism is formed from one or more molecules. FIGS. 1A and 1B each illustrate a crossed wire switch formed from two wires (110 and 130, 140 and 160). Wires 110, 130, 140, and 160 are typically metal or semiconductor nanowires, although in some implementations they can be nanotubes, crossed at some non-zero angle. In between those wires is a layer of molecules or molecular compounds (120 and 150). The particular molecules (125 and 155) that are coupled between two wires to form a junction are identified as switch molecules (RS). For example, when an appropriate voltage is applied across the wires, the switch molecules can be either oxidized or reduced. When a molecule is oxidized (reduced), then a second species is reduced (oxidized) so that charge is balanced. These two species are then called a redox pair. Such redox pairs can be formed using two or more molecules, or using a molecule and one or both of the wires. In general, oxidation or reduction will affect the tunneling distance or the tunneling barrier height between the two wires, thereby altering the rate of charge transport across the wire junction, and serving as the basis for a switch.
While FIG. 1A depicts the use of conventional nanometer-scale wires, FIG. 1B shows wires that include coatings or doped regions 145 and 165. Examples of layers 145 and 165 include modulation-doping coatings, tunneling barriers (e.g., oxides), or other nanoscale functionally suitable materials. Alternatively, the wires themselves may be coated with one or more molecular species such as molecules 150 or some other molecular species.
Thus, electronic devices can formed with sizes from on the order of tens of nanometers to one nanometer simply by making contact between two nanometer-scale wires. Careful selection of the molecules R and/or the outer layers 145 and 165 allows devices with a wide variety of electrical properties.
Examples of interconnects formed through contact (or near contact) of portions of the wires are shown in prior art FIGS. 1C and 1D. As shown in FIG. 1C, nanowires or nanotubes 170 and 180 are also typically arranged so that they cross at some non-zero angle and have a definite separation at the crosspoint. In these structures, at least one of 170 and 180 (usually the “suspended” one, i.e., 170) is typically a nanotube. This scheme can be used to form a suspended array of bi-stable device elements with well-defined OFF and ON states. This array includes a first set of parallel (or roughly parallel) nanotubes or nanowires on a substrate and another set of such wires oriented at a non-zero angle with respect to the first set and suspended on a periodic array of supports. Each crosspoint corresponds to a programmable junction where bi-stability can be envisioned as arising from the interplay of the elastic energy (which produces a potential energy minimum at finite separation) and the attractive van der Waals energy (which creates a second energy minimum when the suspended nanotube/nanowire is deflected into contact or close proximity with the lower wire).
These two minima correspond to well-defined OFF and ON states, respectively. FIG. 1C illustrates the OFF state, i.e., the separated upper-to-lower junction resistance will be very high. FIG. 1D illustrates the ON state where wire 170 has been deflected to make contact with wire 180 (or to be close enough so that the tunneling distance corresponds to a stable energy state). The associated junction resistance in this state will be orders of magnitude lower, providing a well defined ON state.
FIGS. 1A–1D illustrate a number of different nanometer-scale interconnect technologies. In general, all of these technologies, and other technologies not illustrated have the advantage that they are programmable, either once or repeatedly over the lifetime of the device. Many have proposed that cross-wire interconnect arrays based on these nanometer-scale wire technologies and interconnect technologies will be the basis for molecular electronic devices. Moreover, although researchers are optimistic about the ultimate defect rates of such devices, even conservative estimates indicate the need for a high degree of programmability in molecular electronic devices so as to avoid defective portions of the device and make use of redundant circuit elements.
Consequently, many molecular electronic devices will, in some fundamental sense, be analogous to current programmable logic devices. Logic devices, memory devices, and interconnect devices will interoperate to allow circuit designers to achieve the desired architecture or functionality. One important problem that has yet to be adequately addressed by molecular electronics designers and researchers is the design and implementation of devices, e.g., restoring logic, for ensuring that logic signals do not become degraded and/or that those logic signals that do become degraded in some way can be properly restored.
Accordingly, it is desirable to have molecular electronic devices and techniques that address this deficiency in the prior art, yet are relatively easy to design, model, and implement.